1. Field of the Invention
The present invention relates generally to process printing on multi-color presses, and particularly to an improved method of predicting the trap of an overprint of at least two primary colors.
2. Description of Related Art
In wet-on-wet printing on multi-unit presses, it is necessary, in relevant image areas, for ink to transfer or “trap” on top of a previously printed, wet ink film. In an ideal situation, the same amount of ink would transfer to previously printed ink as transfers to plain unprinted paper. However, ink transfer to unprinted paper is usually greater than ink transfer to a previously printed ink film. The amount of ink that actually transfers to a previously printed, wet ink film is commonly referred to as “trap.”
The Preucil trap equation relates the solid ink density for individual inks with the density of the overprint of those inks. The Preucil trap equation assumes that the densities of two overprinted inks will add, and that any deviation from this is solely due to a thinner second layer of ink. The assumption, again, is that ink will not stick as well to ink as to paper.
According to Preucil:
  TRAP  =                    D        OP            -              D        1                    D      2      
where:                DOP is the density of the overprint,        D1 is the density of the first printed ink, and        D2 is the density of the second printed ink.        
Normally trap is used as a press diagnostic. It is measured on press to help determine where a problem might be. For example, if the trap is out of the normal range, then the assumption is that the tack of the inks is not correct, or there is an ink-water balance problem.
In practice, the trap is measured by taking densitometer readings of the overprint color and the two primary colors with the appropriate color filter (red, green and blue) and then relating the densities measured according to the Preucil trap equation. The resulting trap can then be used as a gauge whether adjustment of the ink and/or the press machinery is in order.
One problem surrounding this method of determining the trap is the fact that the formula is complicated, requiring selection of the appropriate color filter (red, green, blue) and taking readings of the overprint color and the two primaries with the same filter. This may result in a less than optimum characterization of at least two of the inks. For example, the green overprint is measured with the red filter and so are the two primaries, cyan and yellow. The red filter is designed to measure cyan ink density and is thus optimum for that primary. But the red filter does not measure the density of the yellow ink at all and only partially reads the green ink density. The result is an approximation to the trap but is rarely exact.
Another problem is that the Preucil trap formula requires knowledge of the order in which the inks making up the overprint have been printed. Such knowledge may or may not be readily available.
Accordingly, an object of the present invention was to provide a statistically valid method for determining the trap in a simple and completely general manner.